Growth-inhibitory and proapoptotic effects of the monoclonal antibody to CD34 molecule, clone 4H11, were tested in CD34+ leukemic cell lines (MOLM-9, JURL-MK1, HEL) and CD34- cell lines (PS-1, ML-2 and CTV-1). We have found that the monoclonal antibody to CD34 inhibited the proliferation and induced apoptosis of all CD34+ cell lines. We did not observe induction of differentiation by anti-CD34 antibody, but a growth arrest of cells in the G0/G1 phase of the cell cycle was detected in all the cell lines studied. Combinations of anti-CD34 antibody with both type I (alpha, beta) or type II (gamma) interferons did not enhance the effects on the cell growth or inhibition of cellular proliferation of the antibody alone. Our data suggest that the monoclonal antibody to CD34 molecule prepared from clone 4H11, after sufficient experimental and preclinical testing on laboratory animals, may provide a new basis for targeted antibody therapy of acute or chronic myeloid leukemia.